Powder coatings for wood, plastics and other heat-sensitive substrates have posed several problems in the quality of the coating formed from the powder. For example, in coating wood substrates, in particular, medium density fiberboard or MDF, moisture in the substrates volatilizes before and during thermal cure of the coating and causes cracking in the resulting coating.
In heat resistant powder coatings, the heat resistant resin may become brittle with use at high temperatures. However, the art does not teach heat resistant coatings having improved elasticity upon use at high temperatures of greater than 200 degrees C.
Core-shell polymers having rubbery core and a hard shell, also known as impact modifiers, have been included into powder coatings to improve impact resistance. However, the impact modifiers in powder coatings have resulted in an undesirably seedy or bumpy appearance, even in films having a thickness of 10 mils or more which use a great deal of powder.
U.S. Pat. No. 6,239,215, to Morita et al., discloses powder coating compositions including a multi layer polymer particle, in which at least one inner layer is a polymer layer having a glass transition temperature (Tg) of not over 20 degrees C. and in which at least one outer layer has a Tg of not less than 60 degrees C., to improve the workability and impact resistance of a coating made from the composition. Morita et al. do not disclose any heat sensitive substrates or heat resistant resins. Further, the multilayer particles from made by Morita et al. agglomerate into large clumps having an average particle size of 200 microns or more. Accordingly, the Morita et al. powders must be coated in thicknesses of at least 200 microns or eight mils just to cover a single layer of the multilayer particles and would, thereby provide bumpy, rough coatings having “seeds” appearing therein.
The present inventors have discovered a powder coating composition which gives unexpectedly superior cracking resistance in coatings for heat sensitive substrates, despite outgassing during cure, as well as elasticity in heat resistant powder coatings. Further, the present inventors have discovered a flexibilized powder coating which provides a smooth coating, even on irregular and uneven surfaces.